In the design of asphaltic paving mixtures for heavy-duty highways one of the important mix properties, and perhaps the least understood, is flexibility, which in part embraces the behavior of paving mixtures under repeated flexure. In a previous paper (1)2 the author defined a number of factors which influence flexibility, grouped as: (1) properties of materials, (2) structural effects, (3) external conditions. At the Soil Mechanics and Bituminous Materials Laboratory at the University of California, a systematic investigation of these factors and their interrelationships is being conducted which may lead to a better understanding of flexibility.